Active safety is becoming more advanced in today's vehicles as the automotive technology is evolving. For example, most new vehicles are equipped with active safety in the form of the well-known ABS brake system which allows a more controlled braking action for the driver of the vehicle.
In a driving situation with a vehicle, the friction between the road and the tire is of high importance since it stipulates the amount of force that can be transferred from the vehicle to the ground. Thus, the friction is a highly relevant parameter for active safety systems when decisions have to be taken related to for example braking and steering with the vehicle. This is relevant both for manually driven vehicles and for autonomous vehicles.
Friction estimation may generally be done by analyzing a tire force versus the so-called slip angle (αslip), see FIGS. 1A-C. The slip angle is the angle between the travel direction Vx of the tire contact patch and the wheel hub direction Wd (i.e., the pointing direction of the wheel). Turning to FIG. 1B which shows the wheel hub direction (Wdh), the travel direction (Vx), an assumed maximum possible parabolic force distribution over the contact area between the tire and the road surface in the lateral direction (y) along x (μhighf(x)), the actual lateral force distribution Fy(x), the resultant lateral force Fy, and the corresponding slip angle for a high friction case (μhigh), and FIG. 1C which shows the wheel hub direction (Wdl), the travel direction (Vx), an assumed maximum possible parabolic force distribution over the contact area between the tire and the road surface in the lateral direction (y) along x (μlowf(x)), the actual lateral force distribution Fy(x) and the resultant lateral force Fy, and the corresponding slip angle for a low friction case (μlow). A comparison between FIGS. 1B-C illustrates that in order to maintain the same resultant lateral tire force Fy magnitude (resulting from the actual lateral force distribution Fy(x) acting on the tire), a larger slip angle is needed for the low friction case (FIG. 1C) compared to the slip angle in the high friction case (FIG. 1B). In other words, when the friction reduces between the tire and the road surface, a larger slip angle is needed to maintain the same lateral tire force Fy, i.e., the driver has to turn the vehicle further. The tire force (i.e., resultant force on the tire) may for example be derived from an inertial measurement unit of the vehicle or from the wheel torque. Further details known in the art can be found in “Tire and vehicle dynamics” (Butterworth-Heinemann, Oxford, 2002) by Hans Pacejka.
US2011/0106458 describes to determine the road surface friction on the basis of the slip angle. One drawback of using the slip angle for road friction estimation is that it requires certain amount of wheel excitation, such as acceleration or cornering, consequently, lack of such events reduces the availability of friction estimation.
Accordingly, there is room for improvements in methods for determining road friction estimates.